1. Field of the Invention
This invention relates to biaxially-oriented multi-layer films, and more particularly to biaxially-oriented multi-layer films which have excellent transparency and low-temperature heat-sealing properties, and which are suited for high-speed automatic packaging.
2. Background Art
Biaxially-oriented multi-layer films such as a biaxially-oriented polypropylene film laminated with a heat-sealing layer, are excellent in various properties such as transparency and rigidity, so that they have been widely used as overwrap-packaging films for foods, tobaccos, cassette tapes, and the like. Because of the recent developments in high-speed automatic packaging machines, quality requirements for suitability of the films for automatic packaging are becoming severer.
The most important properties required for overwrap-packaging films are low-temperature heat-sealing properties. In order to impart good low-temperature heat sealability to the films, there has been developed a method in which a layer of a resin having excellent low-temperature heat-sealing properties, such as a propylene-ethylene random copolymer resin or a propylene-ethylene-butene-1 random copolymer resin, is laminated on a biaxially-oriented polypropylene film. Further, a significant improvement in the low-temperature heat-sealing properties has been made by adding a second component such as polybutene-1 to the above resin layer.
Moreover, in order to meet the recent trend toward high-speed packaging and improve suitability of the films for high-speed packaging by improving the anti-blocking properties of the films, there has been proposed a method in which inorganic fine particles such as of silica or calcium carbonate are added to the above-described resin (Japanese Patent Publication No. 30347/1992); and a method in which organic fine particles such as spherical powder of a silicone resin are added to the above-described resin (Japanese Laid-Open Patent Publication No. 233248/1987).
However, a biaxially-oriented multi-layer film which is prepared by laminating on the surface of a biaxially-oriented polypropylene film a layer of a resin having excellent low-temperature heat-sealing properties, containing inorganic or organic fine particles has been found to have the drawback that when high-speed automatic packaging is conducted with the film for a long period of time, the packaging machine used suffers from staining with the fine particles that fell off the film.
Accordingly, an object of the present invention is to provide biaxially-oriented multi-layer films which have excellent transparency and low-temperature heat-sealing properties and have good suitability for high-speed automatic packaging, and which is free from the trouble of staining on packaging machines.